This invention relates to a target detecting and ranging system. More particularly it relates to a method and apparatus for eliminating unwanted echo signals to obtain and indicate only the desired signals reflected back from targets of interest. This invention minimizes the effect of unwanted echo signals in the indicating apparatus of such systems.
The invention has particular application in radar systems which locate targets by transmitting recurring pulses of electromagnetic energy and receiving the echo pulses caused by the transmitted pulses and reflected back from obstacles. It can also be applied in echo-sounders or the sonar class devices which determine the range and/or the bearing of targets by radiating pulses of ultrasonic energy and receiving the echoes caused thereby.
Hereinafter, the invention will be explained as embodied in a shipborne radar system although the invention is not limited to that particular system.
A radar system ship mounted is required to eliminate echoes reflected back from a rough water surface or echoes from rain-drops (respectively called as "sea clutter" or as "rain clutter") out of the entirety of signals received by an antenna so that the desired signals from targets of interest can, under any weather conditions, be easily and correctly identified on the face of a cathode ray tube indicator. As is well known, if received echo signals including sea clutter signals are directly furnished, without any signal processing, to the indicator, the sea clutter signals appear as a large bright area around the center of the PPI display, thereby overshadowing desired return signals and hence making it difficult to identify them. In order to solve this problem, such well-known methods have been employed as, for example: (1) a FTC (Fast Time Constant) circuit; (2) methods of using filter circuits; and (3) a method employing a STC (Sensitivity Time Control) circuit. However, none of these methods completely solves the problem.
The technique utilizing the STC circuit, for example, can not inherently achieve the object of eliminating only the sea clutter signals from all directions. When the circuit is adjusted as to completely eliminate the sea clutter signals in a specific direction, desired return signals from other directions appear on the PPI display smaller than the ones well representing the actual individual targets. In a worst case condition no desired return signals will appear on the PPI display even if the targets can be recognized by an operator in the surrounding area of the ship. This phenomenon comes from the fact that the intensity of the sea clutter signals from a windward direction is greatest and that from a leeward direction is smaller. Also, from directions therebetween it is the smallest, while the receiver gain control by the STC circuit is constant irrespective of any azimuthal direction being scanned.
The STC circuit functions to restrain the receiver from saturation, thereby suppressing sea clutter signals so as to derive desired return signals. The circuit applies, in synchronism with each transmission of searching pulses, to the first stage of the intermediate frequency amplifier of the reciever negative biasing which automatically varies along a predetermined STC characteristic curve as a function of the time for the time period corresponding to a range. The magnitude of the negative biasing is greatest at zero range and successively increases up to zero at a predetermined range. Hence, if the STC circuit is adjusted to completely eliminate the sea clutter signals from an upwind direction, the negative biasing produced thereby is much greater than necessary for downward and midway directions. Consequently, the return signals from targets of interest lying in the downward and midway directions appear smaller than expected and in some cases will not appear at all on the face of the PPI indicator.